Since polybutylene terephthalate resin has excellent mechanical characteristics, electrical characteristics, heat resistance, weatherability, water resistance, chemical resistance, and solvent resistance, the resin is widely used in various applications as engineering plastics, in automobile parts, electrical and electronic parts, and the like. Such polybutylene terephthalate resin is used in varieties of molded articles as a single component material. It is also known that inorganic filler such as glass fiber is blended in the polybutylene terephthalate resin in some application fields in order to specifically improve the mechanical characteristics.
Although the composition containing an inorganic filler such as glass fiber has high mechanical strength and stiffness, when it is used as thin-walled plate-like or box-like molding articles, for example, micro-switch casing, small coil bobbin, thin-walled connector, and disk cartridge shutter, the composition raises problems such as insufficient molding (insufficient filling in the mold for manufacturing molded article) caused by the decreased flowability resulted from the blending of reinforcing material, and increase in the generation of warp caused by the non-uniform flow of resin. Therefore, there has been wanted a material having improved flowability.
As the methods for improving the flowability of polybutylene terephthalate resin, for example, in Patent Document 1, there is disclosed a polybutylene terephthalate resin composition prepared by blending polybutylene terephthalates having different viscosity (number-average molecular weight) from each other at a specified blending ratio. Patent Document 1 describes that the resin composition improves the cyclic-fatigue resistance of the molded body and gives high flowability in the molten state. The resin composition of Patent Document 1 is, however, inferior in elongation and the like of the resin compared with the case where sole high-viscosity polybutylene terephthalate is used.
Further, in order to improve the flowability, the addition of a flowability-improving agent to polybutylene terephthalate resin is also known. For example, Patent Document 2 discloses a resin composition in which a thermoplastic polyester resin composed mainly of polytetramethylene terephthalate is blended with a specific aromatic polybasic acid ester as a flowability-improving agent. The resin composition described in Patent Document 2, however, shows a tendency of having somewhat inferior mechanical strength compared with the case where the flowability-improving agent is not added.
Other than above, the use of a flowability-improving agent for resin is a common practice to improve the flowability. However, the use of the flowability-improving agent for resin may induce bleeding and separation of surface layer of the molded article, and further a decrease in the mechanical strength is unavoidable.
At the same time, Patent Document 3 discloses a composition containing a fatty acid ester composed mainly of a polyhydric alcohol, selected from glycerin and pentaerythritol, and a fatty acid having 12 or more carbon atoms. In the document, there is a description that the resin composition has excellent molding processability, specifically pellet-capturing property during injection molding, and has excellent moldability. However, Patent Document 3 does not give a description of the flowability of the resin composition at all.
Patent Document 1: JP-A 5-179114 (Claims and Paragraph [0005])
Patent Document 2: JP-A 61-85467 (Claims)
Patent Document 3: JP-A 4-120162 (Claims)